Who do you think is buying .. everything? They're holding substantial fractions of both the whole stockmarket and national debt.
Now... if you mean IRAs then yeah... that's 99% of all private "investors"
It's the corporate businesses that have gotten rid of pensions in favor of 401k plans.
They can also offer some really nice benefits like accessing your pension income at 55 which can be a substantial portion of your last year's salary, and you can keep working elsewhere if you want.